A Model for Nonstoichiometric, Cotranslational Protein Scission in Eukaryotic Ribosomes

The aphthovirus 2A region apparently responsible for the hydrolytic cleavage of a single large polyprotein at a Gly-Pro linkage is only 18 amino acid residues long and is evidently not a proteinase. Here we describe the construction of reporter recombinant polyproteins and provide the results of further mutagenesis experiments designed to test the functions of specific amino acid residues within the foot-and-mouth disease virus (FMDV) 2A region. These results show that a Gly-Pro amide bond is not actually synthesized. The result can be rationalized into a kinetic and structural model for cotranslational aphtho- and cardiovirus polyprotein cleavage in which hydrolysis is mediated by a ribosomally bound 2A polypeptidyl-tRNA molecule at its own 3′-O acyl adenosyl ester linkage. The possible role of the 3-D structure of the 2A polypeptide in preventing peptide bond formation but in allowing the synthesis of the downstream polypeptide sequence is discussed within the context of the new findings.